Primary (non-rechargeable) electrochemical cells having an anode of lithium are known and are in widespread commercial use. The anode is comprised essentially of lithium metal. Such cells typically have a cathode comprising manganese dioxide, and electrolyte comprising a lithium salt such as lithium trifluoromethane sulfonate (LiCF3SO3) dissolved in a nonaqueous solvent. The cells are referenced in the art as primary lithium cells (primary Li/MnO2 cells) and are generally not intended to be rechargeable. Alternative primary lithium cells with lithium metal anodes but having different cathodes are also known. Such cells, for example, have cathodes comprising iron disulfide (FeS2) and are designated Li/FeS2 cells. The iron disulfide (FeS2) is also known as pyrite. The Li/MnO2 cells or Li/FeS2 cells are typically in the form of cylindrical cells, typically an AA size cell or 2/3A Li/MnO2 cell. The Li/MnO2 cells have a voltage of about 3.0 volts which is twice that of conventional Zn/MnO2 alkaline cells and also have higher energy density (watt-hrs per cm3 of cell volume) than that of alkaline cells. The Li/FeS2 cells have a voltage (fresh) of between about 1.2 and 1.5 volts which is about the same as a conventional Zn/MnO2 alkaline cell. However, the energy density (watt-hrs per cm3 of cell volume) of the Li/FeS2 cell is also much higher than a comparable size Zn/MnO2 alkaline cell. The theoretical specific capacity of lithium metal is high at 3861.7 mAmp-hr/gram and the theoretical specific capacity of FeS2 is 893.6 mAmp-hr/gram. The FeS2 theoretical capacity is based on a 4 electron transfer from 4Li per FeS2 to result in reaction product of elemental iron Fe and 2Li2S. That is, 2 of the 4 electrons reducing the valence state of Fe+2 in FeS2 to Fe and the remaining 2 electrons reducing the valence of sulfur from −1 in FeS2 to −2 in Li2S.
Overall the Li/FeS2 cell is much more powerful than the same size Zn/MnO2 alkaline cell. That is for a given continuous current drain, particularly for higher current drain over 200 milliAmp, in the voltage vs. time profile the voltage drops off much less quickly for the Li/FeS2 cell than the Zn/MnO2 alkaline cell. This results in a higher energy obtainable from a Li/FeS2 cell compared to that obtainable for a same size alkaline cell. The higher energy output of the Li/FeS2 cell is also clearly shown more directly in graphical plots of energy (Watt-hrs) versus continuous discharge at constant power (Watts) wherein fresh cells are discharged to completion at fixed continuous power outputs ranging from as little as 0.01 Watt to 5 Watt. In such tests the power drain is maintained at a constant continuous power output selected between 0.01 Watt and 5 Watt. (As the cell's voltage drops during discharge the load resistance is gradually decreased raising the current drain to maintain a fixed constant power output.) The graphical plot Energy (Watt-Hrs) versus Power Output (Watt) for the Li/FeS2 cell is considerably above that for the same size alkaline cell. This is despite that the starting voltage of both cells (fresh) is about the same, namely, between about 1.2 and 1.5 volt.
Thus, the Li/FeS2 cell has the advantage over same size alkaline cells, for example, AAA, AA, C or D size or any other size cell in that the Li/FeS2 cell may be used interchangeably with the conventional Zn/MnO2 alkaline cell and will have greater service life, particularly for higher power demands. Similarly the Li/FeS2 cell which is primary (nonrechargeable) cell can be used as a replacement for the same size rechargeable nickel metal hydride cells, which have about the same voltage (fresh) as the Li/FeS2 cell.
The Li/MnO2 cell and Li/FeS2 cell both require non aqueous electrolytes, since the lithium anode is highly reactive with water. One of the difficulties associated with the manufacture of a Li/FeS2 cell is the need to add good binding material to the cathode formulation to bind the Li/FeS2 and carbon particles together in the cathode. The binding material must also be sufficiently adhesive to cause the cathode coating to adhere uniformly and strongly to the metal conductive substrate to which it is applied.
The cathode material may be initially prepared in a form such as a slurry mixture, which can be readily coated onto the metal substrate by conventional coating methods. The electrolyte added to the cell must be a suitable nonaqueous electrolyte for the Li/FeS2 system allowing the necessary electrochemical reactions to occur efficiently over the range of high power output desired. The electrolyte must exhibit good ionic conductivity and also be sufficiently stable to the undischarged electrode materials (anode and cathode) and to the resulting discharge products. This is because undesirable oxidation/reduction reactions between the electrolyte and electrode materials (either discharged or undischarged) could thereby gradually contaminate the electrolyte and reduce its effectiveness or result in excessive gassing. This in turn can result in a catastrophic cell failure. Thus, the electrolyte used in Li/FeS2 cell in addition to promoting the necessary electrochemical reactions, should also be stable to discharged and undischarged electrode materials.
Primary lithium cells are in use as a power source for digital flash cameras, which require operation at higher power demands than is supplied by individual alkaline cells. Primary lithium cells are conventionally formed of an electrode composite comprising an anode formed of a sheet of lithium, a cathode formed of a coating of cathode active material comprising FeS2 on a conductive metal substrate (cathode substrate) and a sheet of electrolyte permeable separator material therebetween. The electrode composite may be spirally wound and inserted into the cell casing, for examples, as shown in U.S. Pat. No. 4,707,421. A cathode coating mixture for the Li/FeS2 cell is described in U.S. Pat. No. 6,849,360. A portion of the anode sheet is typically electrically connected to the cell casing which forms the cell's negative terminal. The cell is closed with an end cap which is insulated from the casing. The cathode sheet can be electrically connected to the end cap which forms the cell's positive terminal. The casing is typically crimped over the peripheral edge of the end cap to seal the casing's open end.
The anode in a Li/FeS2 cell can be formed by laminating a layer of lithium on a metallic substrate such as copper. However, the anode may be formed of a sheet of lithium without any substrate.
The electrolyte used in a primary Li/FeS2 cells are formed of a “lithium salt” dissolved in an “organic solvent”. Representative lithium salts which may be used in electrolytes for Li/FeS2 primary cells are referenced in U.S. Pat. No. 5,290,414 and U.S. Pat. No. 6,849,360 B2 and include such salts as: Lithium trifluoromethanesulfonate, LiCF3SO3 (LiTFS); lithium bistrifluoromethylsulfonyl imide, Li(CF3SO2)2N (LiTFSI); lithium iodide, LiI; lithium bromide, LiBr; lithium tetrafluorobromate, LiBF4; lithium hexafluorophosphate, LiPF6; lithium hexafluoroarsenate, LiAsF6; Li(CF3SO2)3C, and various mixtures.
Examples of some organic solvents which are referenced in the art for possible use in connection with organic solvents for electrolytes for primary Li/FeS2 cells are as follows: propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), dimethoxyethane (DME), ethyl glyme, diglyme and triglyme, dimethoxypropane (DMP), dioxolane (DIOX), 3,5-dimethlyisoxazole (DMI), tetrahydrofuran (THF), diethyl carbonate (DEC), ethylene glycol sulfite (EGS), dioxane, dimethylsulfate (DMS), 3-methyl-2-oxazolidone, and sulfolane (SU), and various mixtures. (See, e.g. U.S. Pat. No. 5,290,414 and U.S. Pat. No. 6,849,360 B2).
In U.S. Pat. No. 5,290,414 is specifically reported use of a beneficial electrolyte for FeS2 cells, wherein the electrolyte comprises a lithium salt dissolved in a solvent comprising dioxolane in admixture with an acyclic (non cyclic) ester based solvent. The acyclic (non cyclic) ester based solvent as referenced may be dimethoxyethane, ethyl glyme, diglyme and triglyme, with the preferred being 1-2 dimethoxyethane (DME). A specific lithium salt ionizable in such solvent mixture(s) is given as LiCF3SO3 (LiTFS) or Li(CF3SO2)2N (LiTFSI), or mixtures thereof. A co-solvent selected from 3,5-dimethlyisoxazole (DMI), 3-methyl-2-oxazolidone, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and sulfolane.
In U.S. Pat. No. 6,849,360 B2 is specifically disclosed an electrolyte for an Li/FeS2 cell, wherein the electrolyte comprises the salt lithium iodide dissolved in the organic solvent mixture comprising 1,3-dioxolane (DIOX), 1,2-dimethoxyethane (DME), and small amount of 3,5 dimethylisoxazole (DMI).
Thus, it should be evident from the above representative references that the choice of a particular organic solvent or mixture of different organic solvents for use in conjunction with any one or more lithium salts to produce a suitable electrolyte for the Li/FeS2 cell is challenging. This is not to say that many combinations of lithium salts and organic solvents do not produce a Li/FeS2 cell to work at all. But rather the problems associated with such cells using an electrolyte formed with just any combination of lithium salt and organic solvent is that the problems encountered will likely be very substantial, thus making the cell impractical for commercial usage. The history of development of lithium cells in general, whether lithium primary cells, e.g. Li/MnO2, Li/FeS2, or rechargeable lithium or lithium ion cells reveals that just any combination of lithium salt and organic solvent cannot be expected to result in a good cell, that is, exhibiting good, reliable performance.
As an example of a purported advantageous electrolyte mixture the above references reveal advantageous use of dioxolane in combination with an acyclic (non cyclic) ester based solvent, preferably 1,2-dimethoxyethane (DME) to produce an effective electrolyte in conjunction with use of conventional lithium salts. However, dioxolane has the disadvantage of cost and handling.
Accordingly, it is desired to employ solvents for the Li/FeS2 cell electrolyte which are more cost effective and easier to handle than dioxolane. Such solvents are, for example, ethylene carbonate (EC) and propylene carbonate (PC), which are less expensive and easier to store and handle than dioxolane. Ethylene carbonate (EC) and propylene carbonate (PC) alone or in admixture and also in admixture with dimethoxyethane (DME) have produced very suitable solvents for electrolytes for use in connection with Li/MnO2 cells, particularly when the lithium salt for the electrolyte comprises LiCF3SO3 (LITFS). (See, e.g. U.S. Pat. No. 6,443,999 B1)
However, experiments with such electrolytes and electrolyte solvent systems, that is, comprising ethylene carbonate (EC) and propylene carbonate (PC) solvents, while effective in Li/MnO2 cells result in deficiencies when employed, per se, in the context of the Li/FeS2 cell. One of the difficulties is that such ethylene carbonate/propylene carbonate electrolyte solvent mixtures tend to cause or exacerbate the problem of lithium passivation, which normally occurs at least to a degree during the discharge life of the Li/FeS2 cell. Lithium passivation occurs during the Li/FeS2 cell during discharge or storage as a result of gradual reaction with the lithium metal surface in the anode with electrolyte, particularly the electrolyte solvent. A insoluble layer is gradually formed on the lithium metal surface, which tends to passivate the lithium metal surface. Such surface layers, some more debilitating than others, can reduce the rate of the electrochemical reaction involving the lithium anode metal during cell discharge, thus interfering with proper cell performance.
Another problem encountered with the use of ethylene carbonate/propylene carbonate electrolyte solvent mixtures for Li/FeS2 cells is that such solvents tend to cause or exacerbate the problem of initial voltage delay (voltage drop) which may occur typically during an initial phase or initial period of cell usage. Such voltage drop, which can occur at the onset of a new period of cell usage, can reduce the running voltage of the cell for a brief period and thus interfere with attainment of expected consistent, reliable, cell performance. Voltage delay is usually associated with increase of internal resistance of the cell, and usually linked to resistance of the passive layer on the lithium anode.
Accordingly, it is desired to produce a Li/FeS2 cell employing an effective electrolyte therein which reduces or suppresses the rate of lithium anode passivation by preventing or retarding the formation of debilitating passive layer on the surface of the lithium anode.
It is desired to produce a Li/FeS2 cell having an effective electrolyte therein which reduces the amount of voltage delay (voltage drop) occurring at the onset of any new discharge period, or prevents any significant voltage delay from occurring during normal cell usage.
In particular it is desired to produce an electrolyte for the Li/FeS2 cell wherein the electrolyte comprises a cyclic organic carbonate solvent, in particular a cyclic glycol carbonate desirably such as, but not limited to, ethylene carbonate, propylene carbonate, butylene carbonate, and mixtures thereof. (It should be understood that these aforementioned carbonates are cyclic glycol carbonates but they are conventionally referenced in the art as above named ethylene carbonate, propylene carbonate, and butylene carbonate.
It is desired to produce an electrolyte for a Li/FeS2 cell wherein the electrolyte comprises a solvent which is free of dioxolane.